1. Technical Field
The present invention relates to carrying out transmit antenna diversity from a base station to mobile stations and, more particularly, to doing so in a system with both mobile stations equipped to handle antenna diversity and mobile stations not so equipped.
2. Discussion of Related Art
FIG. 4 shows a prior art top-view diagram of the so-called UMTS architecture where the acronym UMTS stands for Universal Mobile Telecommunications System. It includes a core network 10, a UTRAN 20 connected to the core network by a so-called lu-interface and user equipment 40 connected to the UTRAN 20 by a so-called Uu-interface. The acronym UTRAN stands for UMTS Terrestrial Radio Access Network.
FIG. 5 shows a more detailed illustration of the UTRAN of FIG. 1. As shown, the core network may be attached to various radio network subsystems 40, 50, each of which includes a radio network controller interconnected by a so-called lur-interface. Each of the radio network controllers 42, 52 may be connected to a plurality of base stations, called “Node B's” in the third generation. These Node B's are the network elements that have the radio interface (Uu) with the UEs. Antenna transmit diversity allows a same signal to be transmitted via more than one antenna in the same Node B in order to create multipath diversity where it would not otherwise exist. The mobile station receiver combines the plural signals into a composite and is thereby better enabled to perform signal estimation. In the current Third Generation Partnership Project (3GPP) specification defining the functionality of UMTS Terrestrial Radio Access Network (UTRAN), two transmit diversity modes are defined, namely Open Loop Mode including Space Time Block Coding Based Transmit Diversity (STTD) which is an open-loop mode applicable to both common and dedicated channels and Time-switched transmit diversity (TSTD) applicable to the synchronization channel (SCH), and a Closed Loop Mode including Modes 1 (only phase can be adjusted) and 2 (phase and amplitude can be adjusted) applicable to dedicated channels only. A closed loop mode is only used for dedicated channels due to the need for an associated uplink feedback channel for sending adjustment commands to the base station (Node B) to maximize the UE's received power. In the first phase of deployment of 3GPP systems the capacity and performance enhancement offered by transmit diversity was not seen as an important feature by operators, and consequently user equipment (UE) is currently deployed in the field that does not support transmit diversity modes as mandated by the 3GPP specification. This prevents operators from enabling the transmit diversity modes and this could mean significant degradation in service quality for such users which have mobile terminals not supporting the transmit diversity modes. This invention describes how the open loop transmit diversity mode functionality could be modified so that it could be only enabled for UEs supporting it.
For instance, if a UE does not support the STTD transmit diversity scheme as described in the 3GPP specifications and this mode is enabled in a network within which the UE is operating, it will it result in significant degradation in the operation of the UE. Based on the current specification, if the transmit (Tx) diversity is enabled in the network, Tx diversity will be applied at least on a primary common control physical channel (P-CCPCH) and synchronization channel (SCH) and additionally, a common pilot channel (CPICH) shall be transmitted from both antennas using the same channelization and scrambling code but with different pre-defined bit sequences as specified in 3GPP TS 25.211, v 6.0.0 (2003-12), Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD). In practice in order to make the system to work properly Tx diversity would also be applied on the rest of the common channels. If the UE does not support the STTD scheme it is unable to benefit from the transmission from the antenna 2 because it sees its transmission only as interference. This will result in significant loss in demodulation performance and also degrade the performances of initial synchronization and neighbor cell search. Hence, currently the capacity and service quality improvement offered by this transmit diversity mode cannot be utilized before all deployed UEs fully support STTD.